1. Field of the Invention
The present invention relates to inkjet printers, and more specifically to a method for improving print quality by increasing the velocity of the printhead carriage when the temperature of the printhead increases.
2. Description of the Prior Art
Ink-jet printers operate by sweeping a printhead with one or more ink-jet nozzles above a print medium and applying a precise quantity of ink from specified nozzles as they pass over specified pixel locations on the print medium. One type of ink-jet nozzle utilizes a small resistor to produce heat within an associated ink chamber. To fire a nozzle, a voltage is applied to the resistor. The resulting heat causes ink within the chamber to quickly expand, thereby forcing one or more droplets from the associated nozzle. Resistors are controlled individually for each nozzle to produce a desired pixel pattern as the printhead passes over the print medium.
To achieve higher pixel resolutions, printheads have been designed with large numbers of nozzles. This has created the potential for printhead overheating. Each nozzle firing produces residual heat. If too many nozzles are fired within a short period of time, the ink will become less viscous and will eject from the printhead at a higher velocity.
Please refer to FIG. 1. FIG. 1 is a diagram illustrating how an ink drop 12 is ejected from a printhead 10 of the prior art during normal conditions. The printhead 10 is moved across a print medium at a velocity Vp. As the printhead 10 moves across the print medium, the printhead 10 ejects a plurality of ink drops 12 at a drop out velocity Vd. Using vector addition to add the printhead velocity Vp and the drop out velocity Vd, each ink drop 12 is effectively ejected from the printhead 10 with a total velocity V at an angle θ from the vertical. A distance from the printhead 10 to the surface of the print medium is labeled as distance S. From the time that the ink drop 12 is ejected from the printhead 10 to the time that the ink drop 12 reaches the surface of the print medium, the ink drop 12 has traveled a total distance d.
Please refer to FIG. 2. FIG. 2 illustrates operation of the printhead 10 over time during normal conditions. Four different time intervals T1, T2, T3, and T4 are shown in FIG. 2 to show how the ink drop 12 is ejected from the printhead 10 in succeeding time intervals when the ink in the printhead 10 is not excessively heated. Because the temperature of the printhead is at an acceptable level for each of the four time intervals T1, T2, T3, and T4, the velocity V with which the ink drops 12 are ejected is the same for each time interval. That is, the viscosity of the ink in the printhead 10 is substantially constant for each time interval. Since the viscosity is the same in each time interval, the drop out velocity Vd is also the same for each time interval. The velocity Vp with which the printhead 10 moves is kept constant. Therefore, as long as the drop out velocity Vd is kept constant, the distance d that the ink drops 12 are ejected is also the same for each time interval.
Please refer to FIG. 3. FIG. 3 illustrates operation of the printhead 10 over time as the temperature of the printhead 10 rises. In each of the time intervals T1–T4 shown in FIG. 3, the velocity Vp with which the printhead 10 is moving is constant and the distance S from the printhead 10 to the print medium is also constant. However, as the temperature of the printhead 10 increases over the time intervals T1–T4, the viscosity of the ink in the printhead 10 also increases. As a result, the drop out velocity is no longer constant. In time interval T1, the ink in the printhead head 10 is at a low temperature, and the ink drop 12 is ejected with a drop out velocity Vd1 perpendicular from the printhead 10. Combining the velocity Vp of the printhead 10 with the drop out velocity Vd1, the ink drop 12 is effectively ejected from the printhead 10 with a total velocity V1 at an angle θ1 from the vertical. Therefore, the ink drop 12 travels a total distance d1 before reaching the print medium.
As the printhead 10 continues to heat up over time intervals T2–T4, the printhead 10 ejects ink drops 12 at drop out velocities of Vd2, Vd3, and Vd4 respectively. Unfortunately, since the total velocities V2, V3, and V4 are all different from each other in the different time intervals, the distances d2, d3, and d4 that the ink drops 12 travel are also different. This difference in distances leads to a degradation of print quality, as will be shown below.
Please refer to FIG. 4 with reference to FIG. 3. FIG. 4 is a diagram showing degradation of print quality as the temperature of the printhead 10 increases. A total of eight print swaths Swath1–Swath8 are made on a print medium 20 shown in FIG. 4. As indicated by the vertical axis, the print medium 20 is advanced in an upward direction as succeeding print swaths are made. The printhead 10 ejects ink drops 12 onto the print medium 20 as the printhead 10 moves from left to right. Since the temperature of the printhead 10 is increasing with each subsequent print swath, the distance that the ink drops 12 travel from the printhead 10 to the print medium 20 decreases with each subsequent print swath. This is analogous to the decrease in distances d1–d4 over time intervals T1–T4 in FIG. 3. Due to the distances getting shorter with each swath, the image printed on the print medium 20 appears to shift gradually to the left with each succeeding print swath, and print quality suffers as a result.